Although erosion of streambanks and other shorelines by emergent seepage is widespread, this erosion mechanism (termed piping or sapping) has not been recognized as important to the overall erosion process. The mechanism is complex, and interactions with other bank and shore processes tend to mask the effects of piping/sapping. Direct evidence (water emerging from a soil face and carrying away soil particles) is rarely encountered. Several types of indirect evidence are presented and illustrated in this paper, including cavities formed by piping, deposits of dislocated particles below piping zones, blind gullies, staining produced by persistent seepage outflow, and particular types of localized failures (slab toppling, block shearing, and tensile falls caused by undercutting due to piping/sapping). The interactions of this erosion mechanism with other erosion-deposition processes are described. The purpose of this paper is to facilitate identification and evaluation of piping/sapping erosion, particularly for relatively inexperienced field investigators.
